1. Field of the Invention
The present invention relates generally to a wireless communication system and, more particularly, to an interference-aware detection method and apparatus for a User Equipment (UE) in a wireless communication system.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data and multimedia services beyond those of early voice-oriented services. Various mobile communication standards, such as, for example, High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in Institute of Electrical and Electronics Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services. Particularly, LTE is a communication standard that has been developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is an evolved version of LTE, which improves the data transmission capability.
LTE is characterized by 3GPP Release 8 or 9 Capable Base Station and Terminal (UE), while LTE-A is characterized by 3GPP Release 10 Capable Base Station and UE. As a key standardization organization, 3GPP continues standardization of a next release for more improved performance beyond LTE-A.
The existing 3rd and 4th Generation wireless packet data communication systems (such as HSDPA, HSUPA, HRPD, and LTE/LTE-A) adopt Adaptive Modulation and Coding (AMC) and channel-sensitive scheduling techniques to improve transmission efficiency. AMC allows the transmitter to adjust the data amount to be transmitted according to the channel condition. Specifically, the transmitter is capable of decreasing the data transmission amount for a bad channel condition so as to fix the received signal error probability at a certain level, or is capable of increasing the data transmission amount for a good channel condition so as to transmit a large amount of information efficiently while maintaining the received signal error probability at an intended level. The channel sensitive scheduling allows the transmitter to selectively serve a user having a good channel condition among a plurality of users, so as to increase the system capacity as compared to fixedly allocating a channel to serve a single user. This increase in system capacity is referred to as multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method receive partial channel state information that is fed back from a receiver, and apply an appropriate modulation and coding technique at the most efficient time, which is determined depending on the received partial channel state information.
Much research has been conducted to replace Code Division Multiple Access (CDMA) used in the legacy 2nd and 3rd mobile communication systems with Orthogonal Frequency Division Multiple Access (OFDMA) for the next generation mobile communication system. The 3GPP and 3GPP2 are in the process of the standardization of OFDMA-based evolved system. OFDMA is expected to provide superior system throughput as compared to the CDMA. One of the main factors that allows OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, OFDM can be used to obtain more capacity gain using the frequency-varying channel characteristic.
FIG. 1 is a graph illustrating a relationship between time and frequency resources in an LTE/LTE-A system.
As shown in FIG. 1, the radio resource for transmission from the evolved Node B (eNB) to a UE is divided into Resource Blocks (RBs) in a frequency domain and subframes in a time domain. In the LTE/LTE-A system, an RB generally consists of 12 consecutive carriers and occupies 180 kHz bandwidth. A subframe consists of 14 OFDM symbols and spans 1 msec. The LTE/LTE-A system allocates resources for scheduling in subframe units in the time domain and in RB units in the frequency domain.
FIG. 2 is a time-frequency grid illustrating a single resource block of a downlink subframe as a smallest scheduling unit in the LTE/LTE-A system.
As shown in FIG. 2, the radio resource is of one subframe in the time domain and one RB in the frequency domain. The radio resource consists of 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e., 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as Resource Element (RE).
The radio resource structured as shown in FIG. 2 can be used for transmitting many different types of signals, as set forth in detail below.
1. Cell-specific Reference Signal (CRS): reference signal transmitted to all the UEs within a cell
2. Demodulation Reference Signal (DMRS): reference signal transmitted to a specific UE
3. Physical Downlink Shared Channel (PDSCH): data channel transmitted in downlink, which the eNB uses to transmit data to the UE and mapped to REs not used for reference signal transmission in data region of FIG. 2
4. Channel Status Information Reference Signal (CSI-RS): reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell.
5. Other control channels (Physical Hybrid-ARQ Indicator Channel (PHICH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH)): channels for providing a control channel necessary for the UE to receive PDCCH and transmitting ACK/NACK of Hybrid Automatic Repeat reQuest (HARD) operation for uplink data transmission
In addition to the above signals, a zero power CSI-RS can be configured in order for UEs within corresponding cells to receive the CSI-RSs transmitted by different eNBs in the LTE-A system. The zero power CSI-RS can be mapped to positions designated for the CSI-RS, and the UE receives the traffic signal skipping the corresponding radio resource in general. In the LTE-A system, the zero power CSI-RS is referred to as muting. The zero power CSI-RS is mapped to the CSI-RS position without transmission power allocation.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J, according to the number of number of antennas transmitting CSI-RS. Also, the zero power CSI-RS can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs, according to the number of the antenna ports for transmission. For two antenna ports, half of a specific pattern is used for CSI-RS transmission. For four antenna ports, the entire specific pattern is used for CSI-RS transmission. For eight antenna ports, two patterns are used for CSI-RS transmission. Meanwhile, muting is always performed by pattern. Specifically, although the muting may be applied to a plurality of patterns, if the muting positions mismatch CSI-RS positions, it cannot be applied to one pattern partially.
In a cellular system, the reference signal must be transmitted for downlink channel state measurement. In the 3GPP LTE-A system, the UE measures the channel state with the eNB using the CSI-RS transmitted by the eNB. The channel state is measured in consideration of a few factors including downlink interference. The downlink interference includes the interference caused by the antennas of neighbor eNBs and thermal noise, which are important in determining the downlink channel condition. For example, when the eNB with one transmit antenna transmits the reference signal to the UE with one receive antenna, the UE must determine energy per symbol that can be received in downlink and an interference amount that may be received for the duration of receiving the corresponding symbol to calculate Es/Io from the received reference signal. The calculated Es/Io is reported to the eNB such that the eNB determines the downlink data rate for the UE.
In a typical mobile communication system, the base station apparatus is positioned at the center of each cell and communicates with the UE using one or a plurality of antennas deployed at a restricted position. Such a mobile communication system, implemented with the antennas deployed at the same position within the cell, is referred to as a Centralized Antenna System (CAS). In contrast, the mobile communication system implemented with a plurality of Remote Radio Heads (RRHs), belonging to a cell and distributed within the cell area, is referred to as Distributed Antenna System (DAS).
FIG. 3 is a diagram illustrating an antenna arrangement in a conventional distributed antenna system.
FIG. 3 illustrates distributed antenna system-based cells 300 and 310. The cell 300 includes five antennas including one high power transmission antenna 320 and four low power antennas 341, 342, 344, and 343. The high power transmission antenna 320 is capable of providing at least a minimum service within the coverage area of the cell while the low power antennas 341, 342, 343, and 344 are capable of providing UEs with the high data rate service within a restricted area. The low and high power transmission antennas are all connected to the central controller and operate in accordance with the scheduling and radio resource allocation of the central controller. In the distributed antenna system, one or more antennas may be deployed at one geometrically separated antenna position. In the distributed antenna system, the antenna(s) deployed at the same position is referred to as a Remote Radio Head (RRH).
In the distributed antenna system of FIG. 3, the UE receives signals from one geometrically distributed antenna group and regards the signals from other antenna groups as interference.
FIG. 4 is a diagram illustrating interference between antenna groups transmitting to different UEs in the distributed antenna system.
In FIG. 4, a UE1 400 is receiving traffic signal from an antenna group 410. A UE2 420, a UE3 440, and a UE4 460 are receiving traffic signals from antenna groups 430, 450, and 470, respectively. The UE1 400, which is receiving the traffic signal from the antenna group 410, is influenced by the interference of the other antenna groups transmitting traffic signals to other UEs. Specifically, the signals transmitted from the antenna groups 430, 450, and 470 cause interferences to UE1 400.
Typically, in the distributed antenna system, the interferences caused by other antenna groups are classified into two categories:                Inter-cell interference: interference caused by antenna groups of other cells        Intra-cell interference: interference caused by antenna groups of same cell        
In FIG. 4, the UE 1 400 undergoes intra-cell interference from the antenna group 430 of the same cell and inter-cell interference from the antenna groups 450 and 470 of a neighbor cell. The inter-cell interference and the intra-call interference influence the data channel reception of the UE.
Typically, the radio signal received by a UE includes noise and interference. Accordingly, the received signal may be expressed as set forth in Equation (1) below.r=s+noise+interfernee  (1)
In Equation (1), ‘r’ denotes the received signal, ‘s’ denotes the transmitted signal, ‘noise’ denotes the Gaussian distributed noise, and ‘interference’ denotes the interference signal occurring in radio communication. The interference may occur in the following situations.                Interference at serving Transmission Point: When one Transmission Point performs MU-MIMO transmission using a plurality of antennas, the signals addressed to different users may interfere to each other.        Interference from other Transmission Points: The signals transmitted by neighbor cells or neighbor antennas in the distributed antenna system may cause interference to the desired signal.        
A Signal to Interference and Noise Ratio (SINR) varies depending on the size of interference and, as a consequence, affects the reception performance. Typically, the interference is one of the key factors degrading the system throughput of the cellular mobile communication system, and the system throughput is dependent on how to control the interference. In LTE/LTE-A, various technologies are introduced to control interference in association with Coordinated Multi-Point Transmission and Reception (CoMP) as a promised cooperative communication technology. In CoMP, the network integrally controls the transmission of the eNBs and/or transmission points to determine size and presence/absence of interference in downlink and uplink. When two eNBs exist, the central controller of the network may control a second eNB to suspend signal transmission in order to avoid interfering the signal transmitted from a first eNB to the UE.
In the wireless communication system, the error correction code is used to correct an error occurring in transmitting/receiving signals. In the LTE/LTE-A system, convolution code and turbo code are used for error correction encoding. In order to improve the decoding performance of error correction encoding, the receiver performs soft-decision decoding rather than hard-decision decoding when demodulating the symbols modulated with Quadrature Phase Shift Keying (QPSK), 16Quadrature Amplitude Modulation (QAM), and 64QAM. When the sender transmits ‘+1’ or ‘−1’, the hard-decision receiver selects one of ‘+1’ and ‘−1’ to be output. In contrast, the soft-decision receiver outputs the information on whether the received signal is ‘+1’ or ‘−1’ and reliability of the decision. This reliability information can be used for improving decoding performance in the decoding procedure.
Typically, the hard-decision receiver uses Log Likelihood Ratio (LLR) to calculate the output value. When a Binary Phase Shift Keying (BPSK) modulation scheme has a transmission signal that is one of ‘+1’ and ‘−1’, LLR is defined as set forth in Equation (2) below.
                    LLR        =                  log          ⁢                                          ⁢                                    f              ⁡                              (                                                      r                    ❘                    s                                    =                                      +                    1                                                  )                                                    f              ⁡                              (                                                      r                    ❘                    s                                    =                                      -                    1                                                  )                                                                        (        2        )            
In Equation (2), ‘r’ denotes the received signal, and ‘s’ denotes the transmitted signal. The conditional probability density function ƒ(r|s=+1) is the probability density function of the received signal under the condition that the transmitted signal is ‘+1’. Likewise, the conditional probability density function ƒ(r|s=−1) is the probability density function of the received signal under the condition that the transmitted signal is ‘−1’. The LLR can be expressed mathematically in a similar manner even in QPSK, 16QAM, and 64QAM modulation schemes. Typically, the conditional probability density function is used to calculate LLR under the assumption of Gaussian distribution.
FIG. 5 is a diagram illustrating a graph of the conditional probability density function having the Gaussian distribution.
In FIG. 5, reference number 500 denotes the conditional probability density function ƒ(r|s=−1) curve, and reference number 510 denotes the conditional probability density function ƒ(r|s=+1). When the received signal value calculated with the conditional probability density function is determined as denoted by reference number 520, the receiver calculates LLR as log(f2/f1). The conditional probability density function of FIG. 5 corresponds to the case where the noise and interference follow the Gaussian distribution.
In the mobile communication system such as LTE/LTE-A, the eNB transmits a few dozen bits of information to the UE at a time through PDSCH. The eNB encodes the information to be transmitted to the UE, modulates the encoded information in a modulation scheme such as QPSK, 16QAM, and 64QAM, and transmits the modulated signal. If PDSCH is received, the UE generates LLRs for a few dozen encoded symbols to the decoder in demodulating the a few dozen modulation symbols.
Typically, the noise follows Gaussian distribution, but the interference may not follow Gaussian distribution. The reason that the interference does not follow the Gaussian distribution is because the interference is the radio signal addressed to other receivers, unlike the noise. Specifically, since the ‘interference’ in Equation (1) is the radio signal addressed to another receiver, the modulation scheme, such as BPSK, QPSK, 16QAM, and 64QAM, is applied for transmission. When the interference signal is modulated with BPSK, the interference has the probability distribution having ‘+k’ or ‘−k’ value at the same probability. Here, ‘k’ denotes a value determined depending on the signal strength attenuation effect of the radio channel.
FIG. 6 is a diagram illustrating a graph of the conditional probability density function under the assumption that the interference signal is modulated in BPSK modulation scheme in the situation that the received signal is modulated with BPSK. In FIG. 6, it is assumed that the noise follows the Gaussian distribution.
The conditional probability density function of FIG. 6 may be used to observe the function different from the conditional probability density function of FIG. 5. In FIG. 6, reference number 620 denotes the conditional probability density function ƒ(r|s=−1) curve, and reference number 630 denotes the conditional probability density function ƒ(r|s=+1) curve.
Reference number 610 is a size determined according to the signal strength of the interference signal and affected by the radio channel influence. When the received signal value is represented by the conditional probability density function curve 600, the receiver calculates the LLR as log(f4/f3). Since a different conditional probability density function is used, this value differs from the LLR value of FIG. 5. Specifically, the LLR acquired in consideration of the modulation scheme of the interference signal differs from the LLR calculated under the assumption that the interference is Gaussian-distributed.
FIG. 7 is a diagram illustrating a graph of the conditional probability density function under the assumption that the interference signal is modulated in 16QAM modulation scheme while the received signal is modulated in BPSK modulation scheme.
FIG. 7 shows that the conditional probability density function varies according to the modulation scheme applied to the interference signal. In FIG. 7, reference number 700 denotes the conditional probability density function ƒ(r|s=−1) curve, and reference number 710 denotes the conditional probability density function ƒ(r|s=+1). Although the received signals are modulated with BPSK in FIGS. 6 and 7, the interference signal is modulated with BPSK in FIG. 6 and 16QAM in FIG. 7. Specifically, although the same modulation scheme is applied to the received signal, the conditional probability density function changes depending on the modulation scheme applied to the interference signal, and thus the calculated LLR changes too.
As described with reference to FIGS. 5, 6, and 7, LLR has a different value depending on the interference assumed by the receiver. In order to optimize the reception performance, it is necessary to calculate the LLR using the conditional probability density function reflecting the statistical characteristic of the interference in a real situation. Specifically, when the interference signal is modulated in a BPSK modulation scheme, the receiver has to calculate LLR under the assumption that the interference signal has been modulated in BPSK. If the receiver assumes a Gaussian distribution or 16QAM modulation scheme for processing the interference signal modulated in BPSK, a non-optimized LLR value is calculated, resulting in degradation of reception performance.